The present invention relates generally to valves, and more particularly to a live loaded packing assembly specifically suited for sealing the valve stem of a valve to the sleeve or passage in which the valve stem normally moves.
In typical valve construction, a valve stem may undergo a turning or sliding movement, or a combination of both movements, within its sleeve during the process of the valve moving between its open and closed configurations. In this regard, the sealing of the stem must be adequate to contend with such movement, while at the same time ensuring maintenance of fluid tightness against the pressure of the fluid in the valve. A widely used type of stem sealing is a compression packing in which a gland, which may be bolted or threaded, applies a compressive force to a soft compression packing in a stuffer box surrounding a portion of the length of the stem. The resulting radial pressure of the packing onto the stem provides the desired seal so long as the radial pressure exceeds the pressure of fluid in the valve.
In certain valve configurations, compression may be applied to the packing through the use of packing bolts which are each attached at one end to a clamp around the valve body, and at their other end to a spigot, a flange or other projection bearing on, integral with or attached to the gland or sleeve which bears onto the packing. In this particular arrangement, the tightening of the bolts increases the pressure on the packing, thus facilitating the application of radial pressure onto the stem.
In other valve configurations, it is known to attach a spring between the nut used to tighten the bolt and the surface of the spigot or flange. Although coil springs may be used, a conventional practice is to use Belleville springs which are essentially formed as a series of dished washers. Such springs have a higher compression rating than a simple coil spring, with the use of the Belleville springs providing a “live-loaded” packing which can automatically compensate for changes that may take place in the packing under operating conditions of the valve, such as high pressures and temperatures. Since the volume of the packing material may reduce under certain operating conditions, the spring pressure compensates for such reduction and maintains the required pressure, thus avoiding potential harmful effects to the sealing of the stem in an unsprung valve which could result from the reduction in the packing material volume. Alternatively, if the packing volume increases (which can happen with certain packing materials), the radial pressure of the stem in an unsprung valve could increase too much, thus possibly causing sticking of the stem. The spring value, however, can accommodate the pressure increase by means of further compression of the springs.
The “live-loaded” packing construction for a valve described above, while providing a useful amount of self-adjustment to maintain the correct pressure through the packing onto the valve stem, has previously been determined to suffer from certain deficiencies detracting from its overall utility. One such deficiency is the need for longer bolts than would otherwise be required in order to accommodate the springs. The provision of such longer bolts requires sufficient clearance beyond the spigot or gland flange to accommodate such bolts and the corresponding springs, which in turn causes difficulties in fitting a “live-loaded” packing construction to existing valves. This particular deficiency is addressed by the live load valve assembly described in Applicant's prior U.S. Pat. No. 6,622,987 entitled LIVE LOAD ASSEMBLY FOR VALVE issued Sep. 23, 2003, the disclosure of which is incorporated herein by reference.
In U.S. Pat. No. 6,622,987, in one specific embodiment of the improved live-loaded packing construction, an integral spring construction is provided by one or more slots in the arms of the spigot, each such slot extending in a generally radial direction along a plane perpendicular to the axis of the sleeve. Due to the inclusion of the slots therein, the arm is effectively divided into branches which can be forced toward each other as a result of the tightening of a bolt that passes through them. The arm branches, in conjunction with the bolt, act as an integral spring that is capable of accommodating changes in the volume of the packing in a similar manner to that described above in relation to the use of the Belleville springs. In this regard, when the nut is tightened on its corresponding bolt, the branches of the associated arms are forced to deflect toward each other, or one may deflect towards the other, thereby maintaining a stored energy load which is transmitted to the packing. As is further described in U.S. Pat. No. 6,622,987, where this integral spring construction is provided by the slots and the arms of the spigot, the depth of each slot (i.e., the distance between the branches that it separates) may be made oversize to facilitate greater ease in its manufacture. In this oversized condition, to ensure that the correct amount of deflection of a branch toward a corresponding branch occurs, one or more appropriately sized washers or spacers can be fitted around the bolt to reside between the branches and thus reduce the slot depth. The maximum amount of branch deflection can thereby be controlled by selectively varying the size and/or number of spacers between the arm branches.
Though providing improvements over conventional live loaded packing construction, the integral spring construction embodiment described in U.S. Pat. No. 6,622,987 itself suffers from one particular drawback. More particularly, there is a susceptibility in such integral spring construction for an over-torque condition to arise as a result of the branches of each arm being forced too close to each other by the over tightening of the nut on the bolt that passes through them. In this regard, the over tightening the nut as causes one or both of the arm branches to bow or deflect too much toward the other may result in the application of an excessive level of radial pressure onto the valve stem, thus resulting in the sticking of the stem. Though such over-torquing of the nut(s) may be prevented by the placement of the above-described spacers between the branches of each arm in the above-described manner, the fitting of such spacers into the valve is time consuming and cumbersome due to the need to select the appropriate size and number of spacers to prevent an over-flexed condition in the branches from occurring. If such spacers are not used, the tightening of the nuts typically must be facilitated through the use of a torque wrench which is itself time consuming and cumbersome. The present invention, while providing the advantages of the integral spring construction described in U.S. Pat. No. 6,622,987, also eliminates the aforementioned deficiency by providing a live-loaded packing assembly for a valve which is specifically configured prevent the over-torquing condition described above from occurring, yet eliminates the need to use spacers or torque wrenches to tighten the nuts. These and other features and advantages of the present invention will be described in more detail below.